Electronic devices such as computers and automobile components use a heat-dissipating element, such as a heat sink, to dissipate the heat generated by a heating element, such as a semiconductor device or a mechanical component. To make the heat transfer efficient, thermal grease may be applied to fill the space between the heating and heat-dissipating elements.
The thermal grease has a low thermal conductivity compared to the heating element and the heat-dissipating element (typically made of metal); thus, thin layers are advantageous over thick ones. If the purpose is to eliminate any air layer, which has a very low thermal conductivity, from the interface between the heating and heat-dissipating elements, a low-viscosity and high-fluidity thermal grease is more advantageous. For these reasons, it is known to use a low-viscosity thermal grease when the heating and heat-dissipating elements are narrowly spaced. For example, Japanese Unexamined Patent Application Publication No. 2005-330426 states that when the particle diameter of a thermally conductive filler is “greater than 15.0 μm, the silicone grease composition cannot be applied in a sufficiently thin layer and thus is less effective in dissipating heat.”
In recent years, however, many kinds of elements generate heat, and the total heat generation is also increasing. It is thus desired to dissipate heat from multiple electronic elements or from throughout the entire substrate rather than from one particular electronic element. This means that the electronic elements from which heat is being dissipated vary in height, and that a heat-dissipating element may be fit to a diagonally or horizontally positioned heating element in some cases. The forms required for heat dissipation have been diversifying.
A solution to these demands is to use a known thermal grease, usually used as a thin film, in the form of a thick film, but this has the disadvantage of less efficient heat transfer. This is because resin compositions that contain a small-particle-diameter thermally conductive filler can be formed into a thin film for efficient heat transfer, but on the other hand tend to have a low thermal conductivity compared to resin compositions that also contain a large-particle-diameter thermally conductive filler. It is therefore desired to use a thermal grease that contains a thermally conductive filler having a large particle diameter is used as a thick film.
However, thermal greases containing a large-particle-diameter thermally conductive filler have a disadvantage in that the filler easily settles down and separates out of the base oil. For example, Japanese Unexamined Patent Application Publication No. 2012-052137 states, regarding a thermally conductive filler which is preferably 50 μm or less, “Too large an average particle diameter can cause oil separation to proceed readily.” Furthermore, Japanese Unexamined Patent Application Publication No. 2009-221311 states, “An average particle diameter exceeding 30 μm affects the stability of the composition, which makes oil separation more likely to occur.”
A known solution to these problems is to use a thickening or thixotropic additive, such as silica, to prevent the settlement of the thermally conductive filler. This is described in, for example, Japanese Unexamined Patent Application Publication Nos. 2012-052137 and 2009-221311.
Polysaccharides are also known to have an anti-settling effect. Japanese Unexamined Patent Application Publication No. 2002-226819 discloses a technology in which a nonionic and water-soluble cellulose, such as methyl cellulose or ethyl cellulose, is used as an anti-settling agent in an aqueous medium. Japanese Patent No. 3957596 mentions that methyl cellulose is added as a thickener to organopolysiloxane. These are because the cellulose dissolves or disperses in the water medium and “thickens” the system by constructing a weak hydrogen-bond network.
Materials such as amide waxes, polyamides, and urea are also known to construct a weak network of hydrogen bonds or other bonds and prevent settlement by forming such a network structure.
Adding these additives such as silica, however, disadvantageously increases the viscosity of the thermal grease. This translates into that one needs to use a smaller amount of thermally conductive filler to produce a thermal grease with a given viscosity. Moreover, the thermal conductivity of the grease will be low because of the smaller amount of thermally conductive filler.
Attempting to prevent the settlement of a thermally conductive filler according to these existing technologies therefore leads to increasing the viscosity of the composition. Such an attempt is thus considered impracticable in thermal grease and other applications in which high viscosity is unwanted.